The present invention relates to a dynamic mixer.
Dynamic mixers are known which comprise two elements which are rotatable relative to each other about a predetermined axis and between which is defined a flow path extending between an inlet for materials to be mixed and an outlet. In the known mixers, the flow path is defined between surfaces of the elements each of which surfaces has cavities formed within it. Cavities formed in one surface are offset in the axial direction relative to cavities in the other surface, and cavities in one surface overlap in the axial direction with cavities in the other surface. As a result, material moving between the surfaces is transferred between overlapping cavities. Thus, in use, material to be mixed is moved between the elements and traces a path through cavities located alternately on each of the two surfaces. The bulk of the material to be mixed passes through a shear zone in the material generated by displacement of the surfaces. Such mixers incorporating cavities are generally referred to as “cavity transfer mixers”.
Cavity transfer mixers normally have a cylindrical geometry, that is an inner element having a generally cylindrical outer surface and which generally forms a rotor of the device and an outer element having a generally cylindrical inner surface which generally forms a stator of the device. Rows of cavities are formed in the two facing cylindrical surfaces, the rows of cavities overlapping in the axial direction such that material to be mixed generally passes from a cavity in one row of one surface into a cavity in an adjacent row of the other surface. Such conventional cylindrical cavity transfer mixers generally comprise a solid inner rotor which is housed within a split outer stator, it being necessary to manufacture the outer stator in splittable form so as to enable the formation of rows of cavities in the outer stator. The maximum outer diameter of the inner element is less than the minimum inner diameter of the outer element and therefore the mixer can be assembled relatively easily simply by axial insertion of the inner rotor into the outer stator. Given the relative dimensions of the inner and outer elements however an open annular space is defined between the two components.
Problems have been experienced with cylindrical-geometry cavity transfer mixers. In particular, material can pass straight through the annular space defined between the two elements without entering the cavities. This is a particular problem with materials of relatively low viscosity. For example, when materials of dissimilar viscosity are being mixed, materials of relatively low viscosity can effectively short circuit the cavities by travelling straight through the annular space.
A further problem with cylindrical geometry cavity transfer mixers is that asymmetrical transfers can be generated which cause axial back flow or front flow that can generate stagnation patterns with the result that material can become deposited or “hang-up” in the cavities. This is a particular problem when mixing reacting materials and can result in material degradation and uneven flow rates.
Further disadvantageous features of cylindrical geometry cavity transfer mixers is that they are not self pumping or self cleaning. Given that the material flow path through the cavities cannot be directly observed, it is difficult to be sure that material has not become deposited within the cavities. If material does become deposited in one of the cavities, it is difficult to clean out unless the outer element of the structure is split, and even then cleaning is not a simple process.
The formation of cavities on the inner surface of the outer member is difficult to achieve unless the outer member is splittable and as a result manufacturing costs are high. Furthermore, given that the outer element is generally splittable for manufacture and cleaning, leakage can occur through joints in the outer element. These problems have severely, restricted the application of cylindrical geometry cavity transfer mixers.
It is known from for example U.S. Pat. No. 4,680,132 that cavity transfer mixers may have a planar geometry in which the cavities are formed in opposed planar surfaces rather than in opposed cylindrical surfaces. Such a planar geometry makes manufacture of the cavities in the opposed surfaces and cleaning of deposited material from the cavities relatively easier as compared with cylindrical geometries. Problems associated with material bypassing or being deposited within the cavities remain.
It is an object of the present invention to obviate or mitigate the problems outlined above.